A key unresolved question in molecular immunology is the nature of the antigen receptors on thymus-derived lymphocytes (T cells). Recently we used cloned immunoglobulin (Ig) heavy chain mu constant region (C mu) DNA sequences to demonstrate that the C mu gene is expressed as several distinct mu RNA species in mouse thymocytes and in some cultured T cell tumor lines. Our objective is to characterize the T cell mu RNA species, and to analyze rearrangements of the genes encoding them in order to define their relationship to T cell receptors. We will use recombinant DNA technology to construct cloned cDNA copies of the T lymphoma mu RNAs and cloned chromosomal genes. Nucleotide sequence studies on the cloned DNAs should reveal whether the RNAs contain variable regions similar to those which function in B cells. To define which classes of T cells express Ig genes we will examine highly purified normal T cell populations and immunologically functional T cell lines. To gain an insight into the control of Ig gene expression in T and B cells we will construct a variety of T lymphoma cell hybrids and analyze Ig RNA expression in them. We will use a similar recombinant DNA approach in order to ask whether some cells of the granulocyte-macrophage series display immunological diversity, a surprising question raised by our unexpected observation that mu RNA species are present in most myeloid tumor cell lines. These studies should contribute to an understanding of the molecular basis of cell-mediated immune mechanisms and to an understanding of the control mechanisms which result in developmentally distinct transcripts.